1. Field of the Invention
The present invention relates to a diesel engine oil composition, and more particularly, to a diesel engine oil composition suitable for medium-speed diesel engines for marine use and diesel generating power plant.
2. Background Art
In marine diesel engines or diesel engines for power plant, particularly in medium-speed diesel engines, a phenomenon of sudden increase in engine oil consumption has become a issue. The sudden increase phenomenon is attributable to the conditions between rings and liners in the engines; i.e., sticking of the rings by products of engine oil degradation or poor lubrication between the rings and liners. In other words, degradation of engine oils and decrease of lubricity, particularly a decrease in wear resistance of engine oils, play a significant role in the sudden increase of oil consumption. Thus, the service life of an engine oil expires upon occurrence of the sudden increase in oil consumption. Oil consumption may be affected by degradation of specific components in the oil during use, since almost no variation is observed in oil consumption among fresh engine oils. In view that engine oils having long service lives have excellent oxidation stability and wear resistance, development of engine oils which are excellent in both properties is demanded. Japanese Patent Application Laid-Open (kokai) No. 7-197067 discloses an engine oil additive comprising a specific perbasic or neutral sulfonate in combination with a specific polyalkenylsuccinimide and zinc dialkyldithiophosphate. However, the additive has poor characteristics with respect to both oxidation stability and wear resistance.